1. Technical Field of the Invention
This invention relates to an optical device for monitoring or measuring/displaying the arterial oxygen saturation with motion artefact suppression and to a novel medical technique for providing arterial oxygen saturation data.
2. Description of the Prior Art
Monitors are available which use non-invasive optical techniques to measure the arterial oxygen saturation in patients. For example, it is known, that in order to measure blood oxygen saturation, it is necessary to provide a device which passes light through biological tissue, such as the human finger, and to monitor the transmitted or reflected output signal from a photodetector of this device continuously. Such devices are described, inter alia, in International Patent Application No. WO94/03102.
As is well known in the art, these instruments suffer interference due to patient movement, i.e. motion artefact.
Movement of the subject leads to a change in the length of the path of the light through the biological tissue and hence to a variation in the intensity of light received by the photodetector. This renders the device incapable of distinguishing between changes in received light intensity caused by variations in light absorption by the component being monitored (eg oxygen in the blood), and changes in received light intensity caused by variations in the light pathlength due to movement of the subject. The problem is common to all optical monitoring devices and can render these devices inoperative for long periods of time. The problem is particularly severe in critical health care applications, were continuous monitoring is essential.
The device described in WO 94/03102 attempts to address the problem of the motion artefact in measuring SaO2 by using an additional wavelength to enable the motion artefact to be cancelled. Although WO 94/03102 broadly describes the use of a plurality of wavelengths (including the n+1 motion artefact wavelength) the device exemplified uses three wavelengths, namely, a pulse rate wavelength, an SaO2 wavelength and a motion artefact wavelength. However, in practice, the three wavelengths proposed in WO 94/03102 are not sufficient to overcome motion sensitivity.
Generally, medical practitioners desire to measure arterial oxygen saturation (SaO2). For example, conventionally used pulse oximeters measure SaO2. We have now devised an optical measuring or monitoring device which is able to monitor or measure blood oxygen saturation (SO2) and display the arterial blood oxygen saturation non-invasively and to suppress the effects of motion artefact.
Furthermore, existing optical devices do not take into account the variations in transmitted light with varying skin colours. Melanin is present in increasing concentrations from fair through brown to black skin. The peak of its absorption spectrum is at 500 nm decreasing almost linearly with increasing wavelength. Melanin is present in the epidermis, thus, in very high concentrations as is the case in black skin, it can mask the absorption of haemoglobin in the dermis. Even in brown skin, the absorption by melanin is superimposed on that of haemoglobin so that any algorithm which uses the shape of the absorption spectrum in order to produce SO2 value needs to compensate for this fact.